MATERIALS SCIENCE AND ENGINEERING

ADVANCED PHOTOVOLTAIC MATERIALS

Photovoltaic cells are an attractive form of renewable energy, as they have the ability to convert solar radiation directly into electricity without the need for any mechanical moving parts as found in most renewable energy sources (e.g. wind, hydroelectric, geothermal). Additionally, solar cells have virtually no negative environmental consequences (aside from their manufacturing and disposal). Among materials currently utilized for photovoltaic devices, CuInSe₂ (α-CIS) is a leading candidate for the manufacturing of thin film polycrystalline solar cells due to its unique optical and electronic properties. However, the routinely achieved conversion efficiency of solar cells based on α-CIS thin films is still much less than the theoretical conversion efficiency of 24 %, presumably because current deposition techniques (PVD and CVD) generate high densities of electronically active structural defects (dislocations, phase boundaries) in the deposited material.

These difficulties can be explained by the newly established Cu In Se equilibrium phase diagram, the first complete equilibrium phase diagram of the Cu–In–Se system [1-3]. This ternary phase equilibrium diagram indicates that α-CIS is not the only resulting phase that forms on cooling stoichiometric amounts of Cu, In, and Se to grow α-CIS thin films. On the other hand, the phase diagram does indicate that α-CIS can be precipitated directly from a melt, providing that the composition of the melt lies within one of the four compositional phase fields in which α-CIS is the primary phase, shown on the liquidus projection of the Cu-In-Se equilibrium phase diagram in Fig. 1.

Based on the information provided by this liquidus projection, we are investigating the possibility to grow single-phase polycrystalline α-CIS thin films via liquid-phase deposition (LPD). Preliminary results of the implementation of this process have been published [4-5]. Currently, LPD is carried out with the aid of a sliding boat mechanism, often used in the deposition of III–V semiconducting materials. The sliding boat was machined from high-purity graphite (Fig. 2). Verification of the exact liquidus temperature is accomplished by a DTA (differential thermal analysis) instrument. The sliding boat mechanism and DTA where designed, machined, and constructed completely in house. The DTA is run by Labview software from a Windows XP platform. All resulting films are analyzed with the aid of SEM (scanning electron microscopy), XEDS (X-ray energy-dispersive spectroscopy), TEM (transmission electron microscopy), and XRD (X-ray diffractometry).

1. T. Gödecke, T. Haalboom, F. Ernst: Phase Equilibria of Cu–In–Se, I. Stable States and Non-Equilibrium States of the In₂Se₃–Cu ₂Se Subsystem. Zeitschrift für Metallkunde 91 (2000) 622-634.

2. T. Gödecke, T. Haalboom, F. Ernst: Phase Equilibria of Cu–In–Se, II. The Cu–Cu₂Se–In₂Se ₃–In Subsystem. Zeitschrift für Metallkunde 91 (2000) 635-650.

3. T. Gödecke, T. Haalboom, F. Ernst: Phase Equilibria of Cu–In–Se, III. The In₂Se₃–Se–Cu ₂Se Subsystem. Zeitschrift für Metallkunde 91 (2000) 651-662.

4. J. Cowen, L. Lucas, F. Ernst, P. Pirouz, A. Hepp, and S. Bailey: Liquid-Phase Deposition of α-CIS Thin Films. In: Proceedings of Space Photovoltaic Research and Technology Conference (SPRAT), Ohio Aerospace Institute, September 16-18, 2003.

5. J. Cowen, L. Lucas, F. Ernst, P. Pirouz, A. Hepp, and S. Bailey: Liquid-phase deposition of single-phase alpha-copper-indium-diselenide. Materials Science & Engineering, B 116 (2005) 311-319..

This material is based upon work supported by the Department of Energy, National Renewable Energy Laboratory (DOE-NREL). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the DOE-NREL.